In recent years, it has been suggested that a 5-HT5A receptor as one of subtypes of serotonin receptors plays an important role in dementia and schizophrenia. For example, it has been reported that exploratory behavior increases in a 5-HT5A receptor knock-out mouse, and hyperactivity caused by LSD is inhibited in the 5-HT5A receptor knock-out mouse (Neuron, 22, 581-591, 1999). From the results of gene expression analysis, it has been reported that the 5-HT5A receptor is highly expressed in the brain of human beings and rodents, and in the brain, the 5-HT5A receptor is highly expressed in CA1 and CA3 pyramidal cells of the hippocampus involved in memory and in the frontal lobe (cerebral cortex) deeply involved in schizophrenia (Molecular Brain Research, 56, 1-8, 1998). Moreover, it has been reported that the polymorphism of the 5-HT5A receptor gene relates to schizophrenia (Neuroreport 11, 2017-2020, 2000; Mol. Psychiatry 6, 217-219, 2001; J. Psychiatr. Res. 38, 371-376, 2004). Thus, it is suggested that regulation of the function of the 5-HT5A receptor leads to improvement of dementia and schizophrenia. Accordingly, there is a demand for a compound having such a function.
So far, several compounds having affinity with the 5-HT5A receptor have been reported.
For example, it has been reported that a tricyclic compound represented by the following formula (a) binds to the 5-HT5A receptor and is used for treating dementia, schizophrenia, and the like (Patent Document 1).

(In the formula, A represents benzene, thiophene, furan, cyclohexene, or tetrahydropyridine ring, and B represents benzene, cyclohexene, or tetrahydropyridine ring. See the corresponding gazette for more detail.)
The corresponding gazette relates to the tricyclic compound which is different from the tetrahydroisoquinoline derivative of the present invention.
In addition, it has been reported that a bicyclic acylguanidine compound represented by the following formula (b) binds to the 5-HT5A receptor and can be used for treating dementia, schizophrenia, and the like (Patent Document 2).

(In the formula, A represents phenyl or the like; R1, R2, and R3 represent H, lower alkyl, halogen, or the like; R7 and R8 represent H, lower alkyl, or the like; X represents O, S, CR9aR9b; R9a and R9b represent H or the like; a dotted line represents a bond or in existence; m represents 0, 1, or 2; L1 and L2 represent a bond or the like; and R4, R5, and R6 represent H or the like, respectively. See the corresponding gazette for more detail.)
The corresponding gazette relates to a compound in which the moiety of a bicyclic ring group is chroman or benzothiophene, which is different from the tetrahydroisoquinoline derivative of the present invention.
Further, Patent Documents 3 and 4 respectively disclose that naphthoyl guanidine compounds which are represented by the following formulae (c) and (d) and have been substituted with a cyclic group bind to the 5-HT5A receptor and are useful for treating dementia, schizophrenia, and the like.

(In the formula, A represents various cyclic groups including phenyl, pyridyl, and the like. See the corresponding gazette for more detail.)
The corresponding gazette relates to a naphthalene compound which is different from the tetrahydroisoquinoline derivative of the present invention.
In addition, Patent Document 7 discloses that a quinoline or isoquinoline compound which is represented by the following formula (e) and has been substituted with a cyclic group binds to the 5-HT5A receptor and is useful for treating dementia, schizophrenia, and the like.

(In the formula, A represents a cyclic group, and one of Z1, Z2, Z3, Z4, and Z5 represents a nitrogen atom and the others are carbon atoms. See the corresponding gazette for more detail.)
The corresponding gazette relates to a quinoline or isoquinoline compound in which an acylguanidino group binds to a C atom, which is different from the tetrahydroisoquinoline derivative of the present invention.
Moreover, it has been reported that a naphthoyl guanidine compound represented by the following formula (f) is useful as an agent which inhibits Na+/H+ exchange and can be used for treating arrhythmia, angina, and the like (Patent Document 5).

(In the formula, R1 and R3 to R8 represent various substituents via a linker group such as O, amide; or represent H, alkyl, or the like. See the corresponding gazette for more detail.)
In addition, a tetrahydroisoquinoline compound represented by the following formula (g) has been reported as a regulator for 5-HT1B and 5-HT1D receptors (Patent Document 6).

(In the formula, R1 represents alkyl, halogen, or the like; R2 represents aryl, a hetero ring, or carboxamide; W represents a linker group such as —C(O)— or —C(O)NRa—; and ring X represents aryl which may be substituted or a hetero ring which may be substituted. See the corresponding gazette for more detail.)
The compound disclosed in the corresponding gazette essentially has the ring X, which is different from the tetrahydroisoquinoline derivative of the present invention in which an acylguanidino group binds to a N atom in the corresponding moiety.
So far, as a 5-HT5A receptor regulator, a tetrahydroisoquinoline derivative in which an acylguanidino group binds to a N atom has not been reported.